The biggest so far this year has to be Hazard, mainly because of the interest in him and the way he announced it. He's moved to Chelsea (only because they won CL final too), when Barca, Real, Man City, Man United and Arsenal were all publicly interested in signing him. In the end he said "I'll be moving to one of teh Manchester Clubs or Chelsea, I'll announce it on twitter", so there was tons of buzz.
I'm not sure if Hulk has completely signed for Chelsea or not, but this
here says he has, which again will be one of the most high profile signings of the year. Manchester United have signed Shinji Kagawa, the Japanese sensation who has been the top scoring midfielder two years in a row in the Bundesliga and is in the Bundesliga team of the year (most assists too I think) - he's an awesome midfielder.
Barcelona have signed Jordi Alba, the amazing LB you saw for Spain last night for £14m I think. Real Madrid are begging Man City or Spurs to sign either David Silva or Modric. Manchester United is the other club reportedly in for Modric, apparently Modric WILL sign for one of those teams after publicly stating he doesn't want to stay at Tottenham.
Lavezzi has moved from Napoli to PSG (the French version of Man City), Lavezzi is awesome.
That's all from the top of my head, I know I'm forgetting one which is bugging me.